In 1974, statisticians were observed to be convinced that occupational environments of the future would be associated heavily with the rapidly advancing technology of data recovery and data dissemination. Determinations were made that, by 1980, machine writing would essentially occur through the production of paper copies from image screens associated with computers. In particular, by about 1985, operators in the work force would be positioned at display terminals, for example of the cathode ray tube (CRT) variety as opposed to carrying out clerical duties principally associated with the use and maneuvering of paper documents. Such predictions appear to have been accurate, industry now witnessing the production of publications such as newspapers being carried out through keyboard and visual display terminal (VDT) inputs to computerized print-outs, secretarial duties being carried out before visual displays, drafting, process surveillance and governmental functions all evolving within an environment wherein multitudes of operators sit before visual readout terminals to insert and retrieve information. In addition to conventional cathode ray tube readouts, work also is involved with the studying of projected images, for example evolved through data collected with microfilm or microfiche recordation techniques and the like.
Because of the somewhat intensive nature of the work before these visual displays, it has been opined that this evolution of computerization in society has resulted in what may be considered a movement of working environments earlier termed "white collar" toward "blue collar" or toward an industrial character, while the latter industrial form of employment has acquired more of an office character. In this regard, the reader's attention is directed to the following publication:
I. Gunnarson, E., and Ostberg, O. Physical and emotional job environment in a terminal-based data system. Research report 1977:35, OSHA-78-1535, Dept. for Occupational Safety, Occupational Medical Division, AMMF, 1977. PA1 II. Visual Display Units. Reference 139/100/2/2. Dept. of Health, Macarthy Trust Bldg., Lambton Quaz, Wellington, New Zealand. PA1 III. Ward, S. R. Vision and eye effects of video display devices. J. Amer-Optom. Ass. 51, 731-2, 1980. PA1 IV. Laubli, Th., Hunting, W., and Grandjean, E. Visual impairments related to environmental conditions in VDU operators. Dept. of Hygiene and Ergonomies, Swiss Federal Institute of Technology, CH-8092 Zurich, Switzerland. PA1 V. Holler, H., Kundi, M., Schmid, H., Stidl, H. G., Thaler, A., and Winter, N. Arbeitsbeanspruchung and Augenbelastung an Bildschirmgersten, Wien, Verlag des OGB, 1975. PA1 VI. Ostberg, O., Powell, J., and Blomkvist, A. Laser optometry in assessment of visual fatigue in VDU operators. Technical report 1, T, Dept. of Human Work Sciences, Univ. of Lulea, Sweden, 1980. PA1 VII. Guth, S. K., Discomfort glare. Amer. J. Optom. and Arch Amer. Acad. Optom. 38, 247, 1961. PA1 VIII. Potential Health Hazards of Video Display Terminals," U.s. Department of Health and Human Services, DHHS (NIOSH) Publication No. 81-129. PA1 IX. Luckiesch, J., and Guth, S. K. Brightness in the visual field at borderline between comfort and discomfort. Illuminating Engineering 44, 650, 1949. PA1 X. American National Standard Practice for Office Lighting. American National Standards Institute. New York, 1973.
Generally, the visual display terminal before which the operator is seated comprises a display screen, the more prevelant being a cathode ray tube (CRT), a keyboard and associated terminal electronics. For most installations, alphabetical and numerical characters are generated by select phosphor excitation at the raster of the CRT as a dot matrix, usually developed in a 9.times.5 format. The colors generated by these phosphors may vary from a white appearance to various color defining wavelengths, for example, in the green region or orange region. Excitation of CRT phosphors is not steady state in nature but occurs at a predetermined rate, for example, at 40 to 60 scans per second. This intermittency is accommodated by the dwell characteristic of the phosphors which tends to eliminate a flickering effect. Refresh frequencies of character generation should accommodate the critical fusion frequency (CFF) of the operator. The luminance or brightness of the characters generally can be controlled by the operator, however, as luminance is increased, the resolution of the dots becomes diminished. An observed deficiency in character generation at CRT rasters has been associated with a variation in the degree of luminance from the center of the CRT toward the edges. Accordingly, the operator often will not be capable of adjusting the display to achieve a uniform luminance, and thus, uniform readability across a singular given display screen. In connection with the above, the reader's attention is directed to the following publication:
In a survey of visual display terminal operators described in publication I above, it was found that 75 percent of the operators investigated had some visual discomfort in connection with their use of VDTs, 46 percent had severe problems, and 47 percent were observed to have undesirable symptoms which persisted following their work. Similarly, publication II describes a study wherein visual display terminal operators exhibited a significantly higher incidence of asthenopia (eye discomfort) than did a comparison group. These deleterious impedances to the performance and health of VDT operators have become the source of increasing study, as may be evidenced by the following publications:
The above publications consider a variety of observed elements of operator discomfort. For example, subjective eye related problems such as burning sensations, eye muscular fatigue, headaches, itching and eye redness have been reported. Ergonomic related observations have shown other forms of muscular strain to occur, for instance the eyelids of the operator generally are kept open enough to suppress blinking reflexes that occur from time to time and portions of the facial muscles and neck muscles are subjected to the tasks of heavy static holding. Ergonomics broadly is defined as the scientific study of the relationship between man and his working environment.
A number of environmental factors have been determined to contribute to asthenopia, a prominent one being concerned with glare. Glare phenomena are considered to include a broad complex of physiological, psychological and physical factors which combine to determine whether the brightness relationships within a visual environment contribute favorably or unfavorably to seeing conditions. These brightness relationships may influence the visibility of a visual task or their effects may be less obvious and result in decreased ease of seeing. "Direct glare" as is encountered in conjunction with the utilization of visual display terminals generally is caused by viewing an object of low luminance in the presence of considerably higher luminance objects. While a significant reduction in visibility may not be present, nevertheless a feeling of discomfort often is evoked under glare conditions. In this regard, the reader's attention is directed to the following publications:
In the environment of the visual display terminal, the operator generally will be assuming what is considered the most comfortable viewing direction which is forward and somewhat downwardly about 15.degree. to 20.degree. below the horizontal plane of the eye. Thus, the viewing screens of VDTs generally are vertically positioned to accommodate this desired angularity with respect to eye station position. The keyboard usually is positioned beneath the screen at a level associated with the position for carrying out manual writing. As the operator looks away from the screen, gaze generally will be somewhat upward to achieve eye relief and this slightly upward vision will encounter overhead or surrounding environmental illumination as evolved from bright luminares or outdoor illumination through windows and the like. Measurements have shown that the luminances of objects encountered in such viewing are anywhere from 60 to 500 times greater than screen luminance. These luminance ratios exceed those beyond which discomfort is caused and also exceed the recommended standards of the American National Standards Institute. In connection with the above, the reader's attention is directed to publications IV and VII above as well as to the following publications:
Glare also can be occasioned by reflections off of the keyboard or other implements that are in the vicinity of the visual display terminal. Suggestions have been made that the luminance ratio in the central field of vision of the operators be reduced, for example, below 1:3 and a variety of approaches have been used to create a less distracting and less stressful visual environment. For example, overhead luminares have been shielded or utilized with diffusers. Windows have been covered with venetian blinds and the general level of overhead lighting has been lowered. In some instances, polarization filters, micromesh filters or color filters have been positioned in front of the viewing screen to minimize glare. However, such interpositioning has resulted in less than a desirable result. In certain instances, a thin film interference layer has been positioned over the surface of the viewing screens to reduce reflectivity. Further, hoods have been positioned on the screens. However, where the latter approach is made, working posture becomes restricted and adverse anthropometric factors enter into consideration.
The movement of gaze on the part of the operator to a brighter surround following a period of working observation directed into the viewing screen also contributes to asthenopia because of the transient adaptation effects encountered. Whenever the eye's gaze is changed to an object of different illuminance level, a period of time is required to adapt to that luminance level. The greater this difference in luminance or luminance ratio, the longer the operator will take to adapt. Repeated over a working interval, such adaptation requirements can lead to decreased visual efficiency and ocular fatigue.
Another aspect contributing to asthenopia in conjunction with VDT operators is concerned with the concentration of the gaze of the operator upon the viewing screen which gaze will take place under conditions of an eye station to screen distance of between about 40 and 100 centimeters (see publication I above). In order to observe the characters at the screen within this range, the eye of the operator must focus or accommodate and properly converge. This involves the contraction of a muscle (the ciliary body) in the eye which evokes a change in the shape of the crystalline eye lens. Studies have shown that, after two hours of work on the part of an operator at a VDT, the eye becomes under-accommodated for the near working distance. This results in a blurring of near objects. When the worker then looks at a distant object, accommodation is not completely relaxed and the operator remains over accommodated. Such an effect results in distance blur and, additionally, evokes accommodative fatigue. In the above regard, see publications V and VI (supra).
As is apparent, an important factor involved in the development of VDT operator asthenopia is generated in consequence of the relatively high degree of task concentration demanded. Distractions such as visual as well as aural in the vicinity of the operator's immediate environment should be avoided. To this end, VDT installations may be found located within small cubicles or carrels often not appreciated by operators due to a cabin effect. Similarly, hoods or some form of peripheral blinder might be worn by the operators. However, such opaque occlusion of the superior field of view to effect concentration at the inferior field of view generates intolerable confining or claustrophobic reactions.
VDT terminals also are being located within environments other than typically associated with offices. For example, in many instances the terminals are located directly within a factory or manufacturing environment. Not only are the lighting conditions generally less than satisfactory within such environments, particularly for carrying out efficient VDT operations, but also the aspects of environmental safety take on an important role. Usually, within such factory environments, operators are required to wear some form of safety eyewear. In many instances, the requirement for such eyewear militates against solutions to reducing asthenopia occasioned by performance before a VDT terminal.
As indicated above, distractions which may be visual as well as aural should be accommodated for in the region of VDT operation. Avoidance of such visual distraction finds particular application in schools, inasmuch as concentration on subject matter within the school environment can be broken by the distractions of others and can be interfered with by bright sunlight or flickering overhead lights within classrooms. Additionally, although the legibility of reading material may not be affected under present conditions, future classroom student performance utilizing VDTs is inevitable and similar problems to those discussed above in conjunction with adult working environments will be encountered in the educational environment. Further, testing in crowded classrooms often has created security problems for testing monitors.
To the present time, approaches to reducing asthenopia evoked in the VDT occupational environment have been less than satisfactory. In some instances, employee representative organizations have requested relief to the extent of permitting employees one hour of relaxation away from the display screens following each hour of positioning before the screens. A practical and effective development is needed to accommodate this occupational visual impairment in view of the significantly expanding character of data processing and electronic data utilization.